1. Field of the Invention
The present invention relates to a process for producing a grain-oriented electrical steel sheet having a high magnetic flux density, a thin sheet thickness, and an improved watt loss characteristic, which sheet being used for the cores of a transformer and the like.
2. Description of the Related Art
The grain-oriented electrical steel sheet is a soft magnetic material used mainly as the core material of a transformer and other electrical machinery and apparatuses. The magnetic properties required for the grain-oriented electrical steel sheet are an excellent exciting characteristic, which is usually numerically represented by B.sub.8 (the magnetic flux density at a magnetic field intensity of 800 A/m), and an excellent watt loss, which is usually numerically represented by W.sub.17/50 (the watt loss per kg at a magnetization up to 1.7 T and 50 Hz).
The grain-oriented electrical steel sheet is obtained usually by utilizing the secondary recrystallization phenomenon and developing the so called Goss texture having a {110} plane on the steel sheet surface and a &lt;001&gt; axis in the rolling direction. To obtain excellent magnetic properties, it is important to align the &lt;001&gt; axis, which is an easy direction of magnetization, in the rolling direction at a high degree of accuracy. In addition to the orientation, the sheet thickness, the grain size, the resistivity, the surface coating, and the purity of a steel sheet have a great influence on the magnetic properties. The orientation can be drastically enhanced by using methods, in which MnS and AlN are used as the inhibitors and a final cold-rolling is carried out at a heavy draft. In accordance with the enhancement of orientation, the watt loss also can be drastically improved.
Note, the recent large increases in energy costs have forced the manufacturers of transformers to demand more low-watt loss materials for transformers. Amorphous alloys and 6.5% Si steels are being developed as materials having low watt loss but there are problems yet to be solved, concerning their use as transformer materials. A measure for lessening the sheet thickness of a grain-oriented electrical steel sheet promises to lessen the watt loss, because, as known heretofore, such a measure is effective for lessening the eddy current loss, which amounts to 70% or more of the watt loss. Therefore, endeavors have been made to lessen the sheet thickness. The majority of conventional grain-oriented electrical steel sheets are, however, approximately 0.30 mm thick. This thickness was determined by the requirements for assembling the transformer parts. However, together with recent strong demands for saving energy, the need to decreasing the sheet thickness is prevailing over the need to enhance the assembling efficiency, with the result that the transformer manufacturers now tend to use sheets 0.20 mm or less thick. From the point of view of the steel makers, the production of thin gauge-grain-oriented electrical steel sheet involves a problem in that the secondary recrystallization becomes difficult. One reason for this is that a great reduction is necessary for producing thin products from hot-rolled steel sheets having a predetermined thickness, and the texture of the steel sheets is detrimentally influenced by this heavy reduction. This can be eliminated by lessening the sheet thickness of the hot-rolled strips, but this incurs another problem. That is, the finishing temperature of the hot-rolling process is inevitably lowered when the hot-rolled strip has a thin sheet thickness, with the result that the AlN- and MnS- precipitation is promoted and an excessive precipitation size is yielded which is detrimental to the magnetic properties.
Since the measures for lessening the sheet thickness and hence improving the texture are limited, an additional intermediate step must be introduced to the production process. That is, after the hot-rolling, a cold-rolling, an intermediate annealing, and a cold-rolling for reducing the sheet until a predetermined thickness is obtained at a predetermined reduction rate, are successively carried out. In this process, the secondary recrystallization is considerably stabilized and a high magnetic flux density is easily attained. However, this process is unsatisfactory for obtaining products which are 0.18 mm or less in thickness and have improved magnetic properties. One reason for this is that nonhomogeneous regions remain in the structure of the intermediate product and frequently cause linear failure regions in the secondary recrystallization. To overcome drawbacks resulting from nonhomogenity, U.S. Pat. No. 3,632,456 proposes to anneal the hot-rolled strip prior to the first cold-rolling. In this process, the secondary recrystallization is firmly stabilized in products having a sheet thickness as low as 0.14 mm. Such stabilization may be attributable to the high recrystallization degree of the primarily cold-rolled and then annealed sheet, and to a drastic improvement in the structure of the decarburization annealed sheet. The decarburization annealing of the cold-rolled sheet determines the basic structure from which the secondary recrystallization develops. In this process, however, despite the stability of the secondary recrystallization, the magnetic flux density decreases.
Japanese Unexamined Patent Publication No. 58-55530 discloses to decarburize the product at a step later than the hot-rolling and earlier than the completion of final cold-rolling. The magnetic properties are allegedly improved by such an intermediate decarburization. The components of the steels, to which the inventive process of the above publication is applied, are those not using the AlN inhibitor, and the reduction degree at the final cold-rolling is from 40 to 80%.